Mục đích của nghiên cứu này là thay thế một phần xi măng bởi tro bay, một dạng phế thải của nhà máy nhiệt điện. Sự ảnh hưởng của hàm lượng tro bay lên sự phát triển cường độ chịu nén của bê tông được nghiên cứu trong bài báo này. Ba hỗn hợp bê tông thiết kế với 10%, 20% và 30% xi măng được thay thế bởi tro bay so sánh với hỗn hợp bê tông không sử dụng tro bay.
VẬT LIỆU XÂY DỰNG – MÔI TRƯỜNG EFFECT OF FLY ASH CONTENT ON THE COMPRESSIVE STRENGTHDEVELOPMENT OF CONCRETE Dr NGO SI HUY, MEng.LE THI THANH TAM Hong Duc University Dr.HUYNH TRONG PHUOC Can Tho University Abstract: The production and use of ordinary Portland cement in concrete havea significant effect on the surrounding environment by generating a large quantity of carbon dioxide and depletingthe natural resource The objective of this research is to partially replace ordinary Portland cement in concrete mixture with fly ash, which isa byproductfrom thermal power plant The effect of fly ash content on compressive strength development of concrete is investigated Three mixtures were designed with 10%, 20%, and 30% fly ash replacement for cement compared with a control mixture Test results indicate that the workability of fresh concrete increases and the unit weight of concrete reduces with increasing fly ash content The compressive strength of concrete with 10% fly ash is the highest, while that ofconcrete with 30% fly ash is the worst Concrete with 20% fly ash has lower compressive strength than control concrete before 28 days; after 56 days it gets higher Keywords:Ordinary Portland cement, fly ash, workability, concrete mass, compressive strength Tóm tắt:Q trình sản xuất sử dụng xi măng ảnh hưởng lớn đến mơi trường xung quanh hàm lượng khí thải CO2 làm cạn kiệt nguồn tài nguyên thiên nhiên Mục đích nghiên cứu thay phần xi măng tro bay, dạng phế thải nhà máy nhiệt điện Sự ảnh hưởng hàm lượng tro bay lên phát triển cường độ chịu nén bê tông nghiên cứu báo Ba hỗn hợp bê tông thiết kế với 10%, 20% 30% xi măng thay tro bay so sánh với hỗn hợp bê tông không sử dụng tro bay Kết thí nghiệm cho thấy rằng, độ linh động bê tông tươi tăng khối lượng thể tích bê tơng giảm tăng hàm lượng tro bay Hỗn hợp bê tông sử dụng 10% tro bay có cường độ nén cao nhất, hỗn hợp bê tơng chứa 30% tro bay có cường độ nén thấp Cường độ nén hỗn hợp bê tơng sử dụng Tạp chí KHCN Xây dựng – số 2/2017 20% tro bay thấp so với cường độ nén hỗn hợp bê tông không tro bay thời điểm trước 28 ngày tuổi, cao sau 56 ngày tuổi Từ khóa: Xi măng, tro bay, độ linh động bê tông, khối lượng bê tông, cường độ chịu nén Introduction Portland cement concrete is a popular construction material in the world Unfortunately, the production of and use ofordinary Portland cement releases a large amount of carbon dioxide (CO2), which is a major contributor to the greenhouse effect and the global warming of the planet Generally, the production of each ton of cement releases approximately 0.7 ton of CO2 to the environment [1], accounting for around 8% of global CO2 emissions [2] Furthermore, cement production process causes a depletion of thenatural resource Therefore, with concerning the global sustainable development, it is necessary to use supplementary cementitious materials (SCM) as a partial or full replacement of ordinary Portland cement in concrete The most available SCM world-wide is fly ash, a by-product from thermal power plant The effect of fly ash on hardened properties of concrete, especially on compressive strength has received much attention from researchers; however, results are largely different Naik and Ramme (1990) indicated that fly ash could be used to replace up to 40% cement with improved compressive strength [3] Siddique (2003) showed that the use of fly ashas replacement of 40-60% cement in concrete decreased its 28-day compressive strength; however, its 91-day and 360-day compressive strengths were acontinuous and significant improvement [4] Oner et al (2003) [5], Mohamed (2011) [6], and Marthong and Agrawal (2012) [7] found out that the optimum amount of fly ash to replace a part of cement were 40%, 30%, and 20% in their studies, respectively However, Kayali and 31 VẬT LIỆU XÂY DỰNG – MÔI TRƯỜNG Ahmed (2013) reported that replacing a part of cement with fly ash resulted in a reduction in compressive strength of concrete [8] Recent years, Wankhede and Fulari (2014) have shown that concrete with 10% and 20% replacement of cement with fly ash showed better compressive strength at 28 days than that of normal concrete without fly ash; but in the case of 30% replacement, thecompressive strength of concrete decreased [9] On the contrary, Bansal et al (2015) [10] have reported that 10% replacement of cement with fly ash led to a reduction in compressive strengthof concrete; while 20% and 30% replacement resulted in an increase in compressive strength All previous studies mentioned above have different results because fly ash used in each research possessed different physical and chemical properties It is interesting to note that the properties of fly ash concrete are strongly dependent on the characteristic of fly ash used [11] The primary aim of this research is to investigate the effect of raw fly ash content, which is taken from Nghi Son coal power plant as a local material, on compressive strength development of concrete Its effect on fresh concrete properties is also investigated Experimental program 2.1 Material properties Ordinary Portland cement used in this research was Nghi Son Type-PC40 with a compressive strength value of 45 MPa Fly ash was taken from Nghi Son coal power plant The chemical and physical characteristic of cement and fly ash are given in Table According to ASTM C618 (2005) [12] and TCVN 10302 (2014) [13], fly ash used in this research is classified as class-F It is noted that the loss on ignition of fly ash is 15.75% over the requirement of 6% and 12% that stipulated by ASTM C618 (2005) [12] and TCVN 10302 (2014) [13], respectively That is because fly ash used herein is raw material, which is not selected as compared with fly ash used in previous studies [35], where the loss on ignition is lower than 2% This means the quality of fly ash used in this study is worse than that used in previous studies [3-5] The fine aggregate used was natural sand with particle size from 0.15 mm to mm, fineness modulus of 2.67, density of 2.62 T/m3, dry rodded weight of 1.43 T/m3, moisture content of 5.65%, and water absorption capacity of 1.4% The coarse aggregate used was stone with the nominal maximum size of 12.5 mm, density of 2.69 T/m3, dry rodded weight of 1.41 T/m3, moisture content of 0.05%, and water absorption capacity of 0.68% Figure shows the gradation curves for sand and crushed stone Compared with ASTM C33 [14], only the gradation curve of sand is conformed to the requirement for fine aggregate That curve of crushed stone has violated the requirement for the coarse aggregate However, they are existed as local construction materials and does not affect so much to the objective of this research because they are used the same for all mixtures The superplasticizer (SP) of Sikament R7 with a specific gravity of 1.15 is used to reduce water dosage and ensure the desired workability Table Physical and chemical analysis of cement and fly ash Items Physical properties Chemical compositions (%) 32 Cement Fly ash Specific gravity 3.12 2.16 SiO2 22.38 48.38 Al2O3 5.31 20.42 Fe2O3 4.03 4.79 CaO 55.93 2.80 MgO 2.80 1.41 Loss on ignition 1.98 15.76 Tạp chí KHCN Xây dựng – số 2/2017 VẬT LIỆU XÂY DỰNG – MÔI TRƯỜNG 100 Percent passing (%) Percent passing (%) 100 80 60 40 20 Sand Stone 80 60 40 20 0 5 10 11 12 13 Seive size (mm) Seive size (mm) (a) (b) Figure Gradation curve for (a) sand and (b) stone 2.2 Mixture proportions Table Concrete mixture proportions Mixture ID A Fly ash (%) B 10 C 20 D 30 w/b 0.4 Concrete proportion ingredients (kg/m3) Cement Fly ash Sand Stone Water SP 459.3 0.0 867.5 909.4 179.6 4.6 410.7 45.6 861.9 903.5 178.5 4.6 362.7 90.7 856.3 897.6 177.3 4.5 315.3 135.1 850.8 891.9 176.2 4.5 Four concrete mixtures were designed in according with ACI 211.1 [15] with a constant waterto-binder (w/b) ratio of 0.4 The proportion of concrete ingredients is shown in Table Mixture A is a control mixture without fly ash While 10%, 20%, and 30% amount of cement were replaced by fly ash in mixtures B, C, and D, respectively The purpose of these designed mixtures is to investigate the effect of fly ash content on properties of concrete, including concrete unit weight, workability, and compressive strength 2.3 Specimens preparation and test programs The concrete ingredients were mixed in a laboratory mixer The binder materials (cement and fly ash) were first mixed with a part of water for a couple of minutes A portion of SP was then added gradually to the mixture and mixedfor another minutes to achieve a homogeneous paste Then, the sand was added to the paste and the mixer was allowed to run additional minute then addingthe stone, followed by the rest of the mixing water and SP The mixer was run for a further minutes in order to obtain a uniform mixture (a) (b) Figure Concrete specimens(a) after demolding; and (b) curing in water It is noted that this study just only focused on investigating the possibility of using raw fly ash in the production of concrete samples without reinforcement and on evaluating the effect of raw fly ash content on the compressive strength development of the concrete Thus, the effect of raw fly ash with high loss on ignition on reinforcement Tạp chí KHCN Xây dựng – số 2/2017 corrosion will be considered in further research, as well as the application of this type of concrete in any specific area (structural or non-structural elements) will not be discussed in this study Cylindrical concrete specimens with 10 cm in diameter and 20 cm in length were prepared in the 33 VẬT LIỆU XÂY DỰNG – MÔI TRƯỜNG laboratory After one day of casting, they were demolded (as shown in Figure 2a) and immersed in saturated lime-water (as shown in Figure 2b) at a room temperature until the testing age Fresh concrete properties including slump and unit weight were determined The compressive strength of hardened concrete was measured using a controlled compression machine with a loading capacity of 3,000 kN at 3, 7, 14, 28, 56, and 91 days The reported value of compressive strength is the average value of three concrete specimens The measurement of slump and compressive strength of concrete specimens were performed in accordance with ASTM C143 [16] and ASTM C39 [17], respectively.It is noted that the compressive strength values presented herein were converted to equivalent values of cylindrical specimen with 15 cm in diameter and 30 cm in length based on TCVN 3118 (1993) [18] Results and Discussion 3.1 Fresh concrete properties Workability and unit weight of fresh concreteare given in Table The unit weight decreased with increasing fly ash content in theconcrete mixture Since replaced 30% cement by fly ash, concrete unit weight reduced to approximate 3% This is due to the low specific gravity of fly ash in comparison with that of ordinary Portland cement (Table 1) Thus, with the same amount, the volume of fly ash is more than that of cement This leads to a reduction in mass of fly ash concrete specimen as increasing fly ash replacement level On the other hand, workability of fresh concrete increased with increasing of fly ash content Mixture A (without fly ash) and Mixture B (10% fly ash) had the same slump value of 20 mm Further replacing cement with fly ash resulted in increasing workability of fresh concrete When fly ash content increased to 20% (Mixture C), the slump slightly increased to 35 mm The slump of fresh concrete significantly increased to 70 mm since 30% cement was replaced by fly ash (mixture D) This is mainly due to the spherical shape of fly ash particles and its dispersive ability Generally, cement particles have irregular polygonal shape, while fly ash particles have spherical shape with various sizes [19] The spherical shape leads to reduce the friction at the aggregate-paste interface, thus increases the workability of concrete Moreover, the paste volume of fly ash is greater than that of cement because the specific gravity of fly ash is lower than that of cement (Table 1) The increase of the paste volume leads to the increase of plasticity and cohesion, then increase the workability of concrete This finding is in good agreement with previous studies [3,7,20] Table Fresh concrete properties MixtureID A B C D 3.2 Compressive concrete Fly ash (%) 10 20 30 strength development Slump (mm) 20 20 35 70 of The compressive strength development of concrete versus age is presented in Figure As a result, concrete with 10% fly ash (Mixture B) showed the highest compressive strength, while concrete with 30% fly ash (Mixture D) showed the lowest compressive strength Additionally, concrete with 20% fly ash (Mixture C) had lower compressive strength than control concrete (Mixture A) before 28day ages, after 56-day ages it got higher.At day ages, Mixtures A and B (with low fly ash content) had higher compressive strength than Mixtures C 34 Unit weight (T/m ) 2.55 2.52 2.51 2.48 and D (with high fly ash content) The low compressive strength at the early age and the increased strength at the later age of fly ash concrete are associated with the continuous pozzolanic reaction of fly ash in concrete, which only starts significantly after one or more weeks [21] The use of fly ash with optimum dosage increased the compressive strength was proved in previous studies [5,22,23] The main products of cement hydration are calcium silicate hydrate (C-SH) gel and calcium hydroxide (Ca(OH)2) (see equation (1)) While C-S-H is the main carrier of strength in hardened concrete, Ca(OH)2 has Tạp chí KHCN Xây dựng – số 2/2017 VẬT LIỆU XÂY DỰNG – MÔI TRƯỜNG anegative effect on quality of the hardened concrete because of its solubility in water to form cavities and its low strength When fly ash is added, Ca(OH)2 is transformed into thesecondary C-S-Hgel as a result of pozzolanic reaction (see equation (2)) However, if fly ash dosage is added over the optimum value, all of it does not enter into the reaction, it acts as fine aggregate in the mixture rather than a cementitious additive In other word, the fly ash is not used in efficiency Cement hydration: Cement (C3 S , C S ) H O C S H Ca OH 2 (1) Pozzolanic reaction: Ca OH 2 SiO2 C S H (2) As can be seen from Figure 4, the quantity of fly ash used in this study can be replaced upto 20% cement This amount is lower than that in previous published studies (from 40% to 60%) [3-5] That is because fly ash used in this study is araw material with low quality as compared with fly ash used in previous studies [3-5] It means that the optimum fly ash content used in concrete as cement replacement is dependent on its quality Compressive strength (MPa) 50 45 40 35 30 25 A - 0% FA 20 B - 10% FA C - 20% FA 15 D - 30% FA 10 10 20 30 40 50 60 70 80 90 100 Age (Days) Figure Compressive strength development of hardened concrete Conclusions This paper investigates the effect of using raw fly ash taken from Nghi Son coal power plant on the properties of concrete Based on the above experimental results, the following conclusions may be drawn: 1) Increasing fly ash content as an ordinary Portland cement replacement in the concrete mixture resulted in improving the workability of fresh concrete and decreasing its unit weight Since 30% weight of cement was replaced by fly ash, the unit weight reduced to around 3% and workability of concrete increased from 20 mm to 70 mm 2) Concrete with 10% fly ash achieved the highest compressive strength, while concrete with30% fly ash has the lowest compressive strength among all tested concrete Tạp chí KHCN Xây dựng – số 2/2017 3) At the early age, concrete with 20% fly ash exhibited lower compressive strength than control concrete However, itgot higher at the later age of concrete This phenomenon is mainly associated with the continuous pozzolanic reaction of fly ash in concrete 4) Fly ash from this source can be used to replacefor ordinary Portland cement in concrete mixture upto 20% with improved compressive strength REFERENCES [1] World Business Council for Sustainable Development (2009), Cement industry energy and CO2 performance: Getting the right,http://www.wbcsdcement.org/; numbers Visited 17.06.2016 [2] PBL Netherlands Environmental Assessment Agency (2015),“Trend in global CO2 emissions: 2015 Report” 35 VẬT LIỆU XÂY DỰNG – MÔI TRƯỜNG [3] Nail T R., and Ramme B W (1990), “Effect of highlime fly ash content on water demand, time of set, and compressive strength of concrete”, [15] ACI 211.1 (1991):“Standard practice for selecting Siddique R (2003), “Performance characteristics of high-volume class F fly ash concrete”, Cement and Concrete Research, Vol 34, pp 487-493 [5] Oner A., Akyuz S., and Yildiz R (2004), “An concrete containing fly ash and optimum usage of fly ash in concrete”, Cement and Concrete Research, Vol 35, pp 1165-1171 [17] ASTM C39 (2012):“Standard test method for compressive strength of cylindrical concrete specimens” fume on compressive strength of self-compacting Method for determination of compressive strength”, concrete under different curing conditions”, Ain Vietnam Ministry of Science and Technology [19] Papadakis V G (1999), “Effect of fly ash on Marthong C., and Agrawal T P (2012), “Effect of fly Journal of Engineering Research and Applications, Vol 2, No 4, pp 1986-1991 [9] of hydraulic-cement concrete” [18] TCVN 3118 (1993): “Heavyweight concrete – ash additive on concrete properties”, International [8] concrete” Mohamed H A (2011), “Effect of fly ash and silica Shams Engineering Journal, Vol 2, pp 79-86 [7] proportions for normal, heavyweight, and mass [16] ASTM C143 (2015):“Standard test method for slump experimental study on strength development of [6] concrete aggregate” ACI Materials Journal, Vol 87, No 6, pp 619-626 [4] [14] ASTM C33 (2003): “Standard specification for Kayali O., and Ahmed M S (2013), “Assessment of Portland cement systems – Part I Low-calcium fly ash”, Cement and Concrete Research, Vol 29, No 11, pp 1727-1736 [20] Khatib J M (2008), Construction and Building Materials, Vol 22, No 9, Materials, Vol 39, pp 71-76 pp 1963-1971 Journal of Emerging Technology and Advanced Engineering, Vol 4, No 7, pp 284-289 fly self- of performance index”, Construction and Building fly ash on properties of concrete”, International containing of high volume replacement fly ash concrete – concept Wankhede P R., and Fulari V A (2014), “Effect of concrete “Performance compacting ash”, [21] Fraay A L A., Bijen J M., and De Haan Y M (1989), “The reaction of fly ash in concrete a critical examination”, Cement and Concrete Research, Vol 19, No 2, pp 235-246 [10] Bansal R., Singh V., and Pareek R K (2015), “Effect on replacement compressive of fly ash”, partial [22] Memon A H., Radin S S., Zain M F M., and International Journal TrottierJ F (2002), “Effects of mineral and chemical strength with ofEmerging Technologies, Vol 6, No 1, pp 1-6 admixtures on high-strength concrete in seawater”, [11] Bilodeau A., and Malhotra M (2000), “High-volume fly ash system: Concrete solution for sustainable development”, ACI Material Journal, Vol 97, No 1, pp 41-47 Cement and Concrete Research, Vol 32, No 3, pp [12] ASTM C618 (2005):“Standard specification for coal fly ash and raw or calcined natural pozzolan for use in concrete” [13] TCVN 10302 (2014): “Activity admixture – fly ash for concrete, mortar and cement”,Vietnam Ministry of Science and Technology 36 373-377 [23] Papadakis V G., and Tsimas S (2002), “Supplementary cementing materials in concrete – Part I Efficiency and design”, Cement and Concrete Research, Vol 32, No 10, pp 1525-1532 Ngày nhận bài: 29/5/2017 Ngày nhận sửa lần cuối:4/7/2017 Tạp chí KHCN Xây dựng – số 2/2017 ... reinforcement and on evaluating the effect of raw fly ash content on the compressive strength development of the concrete Thus, the effect of raw fly ash with high loss on ignition on reinforcement... note that the properties of fly ash concrete are strongly dependent on the characteristic of fly ash used [11] The primary aim of this research is to investigate the effect of raw fly ash content, ... high fly ash content) The low compressive strength at the early age and the increased strength at the later age of fly ash concrete are associated with the continuous pozzolanic reaction of fly ash